Provided by: manpages_2.17-1_all bug

NAME

       udp - User Datagram Protocol for IPv4

SYNOPSIS

       #include <sys/socket.h>
       #include <netinet/in.h>
       udp_socket = socket(PF_INET, SOCK_DGRAM, 0);

DESCRIPTION

       This  is  an  implementation of the User Datagram Protocol described in
       RFC 768.  It implements a connectionless,  unreliable  datagram  packet
       service.   Packets  may  be reordered or duplicated before they arrive.
       UDP generates and checks checksums to catch transmission errors.

       When a UDP socket is  created,  its  local  and  remote  addresses  are
       unspecified.   Datagrams  can  be  sent  immediately using sendto(2) or
       sendmsg(2) with a valid  destination  address  as  an  argument.   When
       connect(2)  is  called on the socket the default destination address is
       set and datagrams can now be sent using  send(2)  or  write(2)  without
       specifying  an  destination  address.   It is still possible to send to
       other destinations by passing an address to  sendto(2)  or  sendmsg(2).
       In order to receive packets the socket can be bound to an local address
       first by using bind(2).  Otherwise the socket layer will  automatically
       assign   a   free   local   port   out   of   the   range   defined  by
       net.ipv4.ip_local_port_range and bind the socket to INADDR_ANY.

       All receive operations return only one  packet.   When  the  packet  is
       smaller than the passed buffer only that much data is returned, when it
       is bigger the packet is  truncated  and  the  MSG_TRUNC  flag  is  set.
       MSG_WAITALL is not supported.

       IP  options  may be sent or received using the socket options described
       in ip(7).  They are only processed by the kernel when  the  appropriate
       sysctl  is enabled (but still passed to the user even when it is turned
       off). See ip(7).

       When the MSG_DONTROUTE flag is set on sending the  destination  address
       must refer to an local interface address and the packet is only sent to
       that interface.

       UDP fragments a packet when its total length exceeds the interface  MTU
       (Maximum Transmission Unit).  A more network friendly alternative is to
       use path MTU discovery as described in the IP_MTU_DISCOVER  section  of
       ip(7).

ADDRESS FORMAT

       UDP uses the IPv4 sockaddr_in address format described in ip(7).

ERROR HANDLING

       All  fatal  errors  will  be passed to the user as an error return even
       when the socket is not connected.  This  includes  asynchronous  errors
       received  from  the network. You may get an error for an earlier packet
       that was sent on the same socket.  This  behaviour  differs  from  many
       other BSD socket implementations which don’t pass any errors unless the
       socket is connected.  Linux’s behaviour is mandated by RFC 1122.

       For  compatibility  with  legacy  code  it  is  possible  to  set   the
       SO_BSDCOMPAT  SOL_SOCKET  option to receive remote errors only when the
       socket has been connected (except for  EPROTO  and  EMSGSIZE).   It  is
       better  to  fix  the code to handle errors properly than to enable this
       option.  Locally generated errors are always passed.

       When the IP_RECVERR option is enabled all  errors  are  stored  in  the
       socket  error  queue  and  can  be  received  by  recvmsg(2)  with  the
       MSG_ERRQUEUE flag set.

SOCKET OPTIONS

       To set or get a UDP  socket  option,  call  getsockopt(2)  to  read  or
       setsockopt(2) to write the option with the option level argument set to
       IPPROTO_UDP.

       UDP_CORK (since Linux 2.5.44)
              If this option is enabled, then all data output on  this  socket
              is  accumulated  into a single datagram that is transmitted when
              the option is disabled.  This option should not be used in  code
              intended to be portable.

IOCTLS

       These ioctls can be accessed using ioctl(2).  The correct syntax is:

              int value;
              error = ioctl(udp_socket, ioctl_type, &value);

       FIONREAD (SIOCINQ)
              Gets  a  pointer  to an integer as argument. Returns the size of
              the next pending datagram in the integer in bytes, or 0 when  no
              datagram is pending.

       TIOCOUTQ (SIOCOUTQ)
              Returns  the  number of data bytes in the local send queue. Only
              supported with Linux 2.4 and above.

       In addition all ioctls documented in ip(7) and socket(7) are supported.

ERRORS

       All  errors documented for socket(7) or ip(7) may be returned by a send
       or receive on a UDP socket.

       ECONNREFUSED No receiver was associated with the  destination  address.
       This might be caused by a previous packet sent over the socket.

VERSIONS

       IP_RECVERR is a new feature in Linux 2.2.

CREDITS

       This man page was written by Andi Kleen.

SEE ALSO

       ip(7), raw(7), socket(7)

       RFC 768 for the User Datagram Protocol.
       RFC 1122 for the host requirements.
       RFC 1191 for a description of path MTU discovery.